Oxidative dehydrogenation catalyst

ABSTRACT

Hydrocarbons are oxidatively dehydrogenated in the presence of a catalyst comprising a mixture of oxides of manganese, phosphorus, and an alkali metal, optionally, supported on a refractory oxide. In one embodiment, ethane is converted to ethylene in the presence of a catalyst comprising a mixture of oxides of manganese, phosphorus, and sodium supported on alumina.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of our copending application, Ser. No.149,601, filed May 13, 1980, now U.S. Pat. No. 4,310,717 entitledOXIDATIVE DEHYDROGENATION AND CATALYST.

The present invention relates to catalytic compositions and to chemicalconversion processes using the catalyst. In accordance with anotheraspect, this invention relates to a process for the oxidativedehydrogenation of hydrocarbons in the presence of a catalyst comprisingmanganese, phosphorus, and an alkali metal with, or without, arefractory support. In accordance with a further aspect, this inventionrelates to the oxidative dehydrogenation of paraffinic hydrocarbons tothe corresponding mono-olefins in the presence of a catalyst comprisinga mixture of oxides of manganese, phosphorus, and an alkali metal. Inaccordance with a further aspect, this invention relates to a catalyticcomposition comprising a mixture of oxides of manganese, phosphorus, andan alkali metal with, or without, a refractory support.

BACKGROUND OF THE INVENTION

It is a continuing goal of the chemical processing industries to findboth primary and alternative methods of converting raw materials, whichare readily available, into other materials which may be less plentifuland more valuable. Some of the more useful of such conversion methods isdehydrogenation processes for the conversion of organic compounds, suchas hydrocarbon feedstocks to unsaturated compounds. A number ofcatalytic processes have been developed which have attained some measureof commercial success. There is a continuing search to develop catalyticmaterials and processes which are more efficient in minimizing sidereactions, improving conversion rates, improving yields andselectivities to desired end product, or which have a low susceptibilityto deactivation, e.g., are capable of extended periods of operationwithout regeneration and/or which can be readily regenerated to anactivity approaching that of fresh catalysts. The present inventionrelates to a novel catalyst composition which is useful in the oxidativedehydrogenation of hydrocarbons.

Accordingly, an object of this invention is to provide an improvedprocess for the oxidative dehydrogenation of hydrocarbons.

Another object of this invention is to provide a novel catalystcomposition.

A further object of this invention is to provide a catalytic compositionuseful in the oxidative dehydrogenation of hydrocarbons.

Other aspects, objects, and the several advantages of this inventionwill become apparent to those skilled in the art upon reading thisdisclosure and the appended claims.

The present invention provides a novel catalyst and a novel process forthe conversion of hydrocarbon feedstocks to hydrocarbons having agreater degree of unsaturation and which have the same and sometimeslower number of carbon atoms as in the hydrocarbon feed.

In accordance with the invention, hydrocarbons and, particularly,paraffinic hydrocarbons are oxidatively dehydrogenated to unsaturatedproduct or products having a greater degree of unsaturation bycontacting such hydrocarbons under dehydrogenation conditions in thepresence of a catalyst mixture comprising manganese, phosphorus, and analkali metal with, or without, a refractory support.

In accordance with one embodiment of the invention, light paraffinichydrocarbons are oxidatively dehydrogenated to the correspondingmono-olefins by contacting same under oxidative dehydrogenationconditions in the presence of a catalyst mixture of oxides of manganese,phosphorus, and alkali metal with, or without, a refractory support.

In accordance with the invention, paraffinic hydrocarbons can beconverted in good yields to mono-olefins. The invention is particularlysuitable for the conversion of ethane to ethylene, butane to butene,isopentane to isoamylene, and the like.

Further, in accordance with the invention, a novel catalyst is providedcomprising a mixture of oxides of manganese, phosphorus, and an alkalimetal with, or without, a refractory support.

CATALYST

The catalyst of this invention comprises manganese oxide, phosphorusoxide, and at least one element from Group Ia of the Periodic Table,also as the oxide. The members of this group are lithium, sodium,potassium, rubidium, and cesium. The atomic ratio in whichmanganese:phosphorus:alkali metal elements are combined to form thecatalyst is 1:0.3 to 0.25:0.06 to 0.30, respectively. Preferably theratio, expressed in the same manner and in the same order, is 1:0.05 to0.20:0.10 to 0.30.

Although it is not necessary, it is preferable for the catalyst to beprepared on a refractory oxide that is suitable for a support, e.g.,activated alumina, magnesia, zinc aluminate, and the like. The atomicratio of manganese to metal atoms in the support can range from about0.6 to about 1.5, preferably the ratio is about unity.

A suitable method for preparing the supported catalyst is to impregnatethe support (of suitable particle size) alternately with solutions ofmanganese and of alkali metal phosphate by the method in incipientwetness. After each impregnation, the preparation is dried in an oven toremove the solvent. After addition of the catalyst to the support hasbeen completed, the catalyst is prepared for use by calcining in air atabout 800° C. for three hours.

Unsupported catalyst is conveniently prepared by mulling a suitablemanganese compound with a solution that contains both phosphorus and thealkali metal until a smooth, uniform slurry-like product has beenobtained. This is dried in an oven to remove water, then calcined in airat about 800° C. for three hours.

For preparation by impregnating a porous, refractory oxide manganousnitrate, Mn(NO₃)₂, is preferred. Unsupported catalysts can be made frommanganous acetate, manganese carbonate MnCO₃, or manganese dioxide MnO₂.Phosphorus and the alkali metal M can be incorporated from solutions oforthophosphoric acid and the alkali compounds MOH, M₂ CO₃, MHCO₃, andthe like. Or, for convenience, compounds which contain both elementssuch as the orthophosphates MH₂ PO₄, M₂ HPO₄, and M₃ PO₄, and thepyrophosphate M₄ P₂ O₇ can be used.

OXD Process

Oxidative dehydrogenation (OXD) of paraffins to olefins using thecatalyst of this invention is believed to involve the reaction of twohydrogen atoms from the paraffin with an atom of oxygen from thecatalyst to produce, in addition to an olefin molecule, a molecule ofwater. The process is effected in a cyclic manner in which the catalystis contacted alternately with a paraffinic hydrocarbon, then with a freeoxygen-containing gas such as air.

Paraffins amenable to dehydrogenation range from ethane to dodecanealthough it is preferable to use the lighter paraffins ranging fromethane to pentane. These can be branched or unbranched. The use of thiscatalyst to convert ethane to ethylene is particularly preferred. Somespecific examples of other feeds include propane, butane, isobutane,pentane, octane, dodecane, 2-methylhexane, 2,4-dimethyloctane, and thelike, including mixtures thereof. Heavier feedstock can be treated atthe same pressures and residence times that are suitable for ethane, butthe temperature for their treatment generally decreases with increasingcarbon number within the temperature range described below.

The dehydrogenatable feedstocks can be converted, according to theprocess of the present invention, under any suitable conditions so longas conditions are such to oxidatively dehydrogenate the hydrocarbon orhydrocarbons to mono-olefins. Thus, the conditions of temperature,pressure, and period of contact will vary depending upon the particularfeedstock and catalyst combination but, in any event, the conditions aresufficient to dehydrogenate the paraffinic hydrocarbon to thecorresponding mono-olefin.

The temperature for oxidative dehydrogenation can range between about600°-800° C. For ethane, the preferred temperature ranges between about675°-740° C. Catalyst activity is retained longer by starting to use apreviously unused catalyst at temperatures lower than these, e.g., about550° C., then increasing to the range stated here during the first fewprocess cycles.

Below pressures at which the olefin product begins to polymerize OXDconversion is not greatly affected by reaction pressure. Suitablepressure for operation is in the range of 10 to 520 kPa; preferably thereaction is run at about 100-200 kPa.

Reactant feed rate, expressed as volumes of gas at standard conditionsper volume of catalyst per hour (GHSV) can range between 200-1000; arate between about 400-600 is preferred.

Of considerable importance to the OXD process is the length of theprocess cycle in which paraffins are contacted with the catalyst beforethe catalyst is regenerated. At the conclusion of the regenerationcycle, the manganese in the catalyst is believed to be present as Mn₃O₄. The quantity of paraffins that is contacted with catalyst before thenext regeneration period preferably will not exceed that which willconvert all Mn₃ O₄ to MnO, i.e., the number of moles of hydrocarbon fedin one process cycle will not exceed the number of moles of Mn₃ O₄ inthe catalyst zone. Although MnO is not readily reduced further it willbe ineffective to dehydrogenate additional feed and, in addition,becomes more difficultly regenerable.

Although it is not required, prudent operation may dictate a purge ofthe reactor with an inert gas, e.g., nitrogen or carbon dioxide, betweendehydrogenation and regeneration cycles to avoid mixing hydrocarbonswith free oxygen.

Preparation of some catalysts and results of their use to convert ethaneto ethylene are shown in the following examples.

EXAMPLE I

Catalyst A was a supported catalyst prepared by impregnating activatedalumina. 100 g of Filtrol 86 (Trademark) alumina was impregnated with484 g (1.35 moles) of 50% Mn(NO₃)₂ using sixteen 30 g portions. Aftereach addition, the catalyst was dried at 150° C. for 6-10 hours beforethe next step. A total of 19 g (0.043 moles) of Na₄ P₂ O₇.10H₂ Odissolved in 60 mL of water was added--half when one half of themanganese had been added and the other half when only one portion ofmanganese nitrate remained to be added. After impregnation had beencompleted, catalyst A was dried at 150° C. for 24 hours, then calcinedin air at 704° C. for three hours. The atomic ratio Mn:P:Na in catalystA was calculated to be 1:0.10:0.20.

Catalyst B was prepared from catalyst A by adding one gram of Na₄ P₂O₇.10H₂ O to 30 g of the latter by impregnation with aqueous solution;then drying and calcining in air as for catalyst A. The atomic ratioMn:P:Na in catalyst B was calculated to be 1:0.14:0.28.

Catalysts C, D, E, and F were prepared from different manganesecompounds and contained no catalyst support. All had the same atomicratio Mn:P:Na=1:0.067:0.133. They were made as follows. Catalyst C wasprepared by adding excess ammonium hydroxide to 107 g manganous nitratedissolved in about one liter of water. After heating to remove most ofthe ammonia, it was filtered, and 8.94 g Na₄ P₂ O₇.10H₂ O dissolved in aminimum volume of water was mixed thoroughly with the filter cake.Catalyst D was prepared by mixing 69.0 g of powdered manganese carbonatewith 60 mL of solution containing 8.92 g of Na₄ P₂ O₇.10H₂ O. Catalyst Ewas prepared by dissolving 147 g of Mn(C₂ H₃ O₂)₂.4H₂ O and 8.92 g Na₄P₂ O₇.10H₂ O in about 600 mL of water and heating on a hot plate untilthe volume of solution was reduced to about 100 mL. Catalyst F wasprepared by mixing 52 g of manganese dioxide powder with about 60 mL ofsolution containing 8.9 g of Na₄ P₂ O₇.10H₂ O. Ten grams of powderedstarch was then added to the mixture. Preparation of catalysts C throughF was completed by drying the mixtures described in an oven at about120° C., then calcining them in air at 820° C. for three hours.

EXAMPLE II

Supported catalysts A and B were used in runs to produce ethylene fromethane by OXD. Fifteen mL portions of -16+40 mesh catalyst were placedin a quartz reactor in a temperature controlled furnace and contactedalternately with ethane at 400 GHSV for three minutes, then with anequal volume mixture of air plus nitrogen, each flowing at 1200 GHSV,for six minutes. The runs continued for 900 cycles, and results ofanalyses on samples taken during that period are presented in Table I.Analyses were made on snap samples taken 1.5 minutes into the processperiod.

                  TABLE I                                                         ______________________________________                                                  A           B                                                               Temp.   Conv.,  Selectivity,                                                                          Conv.,                                                                              Selectivity,                            Cycle   °C.                                                                            %       %       %     %                                       ______________________________________                                        1       650     39      54      27    63                                      2       650     41      62      45    65                                      3       700     43      70      54    75                                      10      700     45      70      55    80                                      425     700     37      84      52    78                                      650     700     23      86      --    --                                      765     700     --      --      16    90                                      900     700     22      93      16    96                                      ______________________________________                                    

During the first few cycles the activity and selectivity of bothcatalysts increased. Then, as the catalysts aged further, activitytended to decline while the selectivity to ethylene continued toincrease.

Unsupported catalysts C through F were used in runs made in a somewhatdifferent manner. Two mL portions of -16+40 mesh catalyst in quartzreactors in a temperature controlled furnace were contacted as follows.Nitrogen flowed continuously at 500 GHSV. During the four minuteregeneration cycle air at 2500 GHSV was combined with the nitrogen. Flowof air was discontinued for two minutes to provide a nitrogen purge,ethane was then combined with nitrogen, also at 500 GHSV, for a 2.5minute OXD period. Flow of ethane was discontinued for two minutes toprovide another nitrogen purge, then regeneration was repeated. Threesnap samples from the OXD portion of each cycle were collected andanalyzed. Table II records the results of the average of these threeanalyses for catalysts C-F, and also using corundum only. All runs weremade at 703° C.

                  TABLE II                                                        ______________________________________                                        Catalyst  Cycle     C.sub.2 H.sub.6 conv., %                                                                  Sel. to C.sub.2 H.sub.4,                      ______________________________________                                                                        %                                             C         1         97.2        50.9                                                    5         97.4        70.3                                                    127       38.3        69.8                                          D         1         75.0        75.7                                                    5         57.4        85.1                                                    127       14.4        93.6                                          E         1         35.7        86.3                                                    5         33.5        88.0                                                    127       7.5         93.9                                          F         1         31.9        87.9                                                    5         22.5        89.3                                                    127       5.9         91.2                                          Corundum  1         19.8        90.1                                                    5         18.0        92.1                                                    127       14.7        93.0                                          ______________________________________                                    

These unsupported catalysts are also active and selective, particularlyduring the first process cycles, to convert ethane to ethylene byoxidative dehydrogenation.

We claim:
 1. A catalyst composition consisting essentially of a mixtureof oxides of(a) manganese, (b) phosphorus, and (c) an alkali metalwherein the atomic ratio of (a):(b):(c) is 1:0.03 to 0.25:0.06 to 0.30.2. A catalyst according to claim 1 which additionally, comprises arefractory oxide support.
 3. A catalyst according to claim 2 whereinsaid refractory oxide support is selected from the group consisting ofactivated alumina, magnesia and zinc aluminate; and wherein the atomicratio of manganese to metal atoms in the support is within the range ofabout 0.6 to about 1.5.
 4. A catalyst according to claim 3 wherein (c)is sodium.
 5. A catalyst according to claim 4 wherein said refractoryoxide support is activated alumina.
 6. A catalyst according to claim 3wherein (c) is sodium and wherein the atomic ratio of (a):(b):(c) is1:0.05 to 0.20:0.10 to 0.30.